First argument:

AllRats Create rat lines for all loaded nets that aren't already connected on with copper.
SelectedRats Similarly, but only add rat lines for nets connected to selected pins and pads.
Close Selects the shortest unselected rat on the board.

The second argument is temporary and optional, for a workaround of a bug in the old (to be removed) autorouter. If the value of the second argument is "manhattan", non-axis-aligned lines are ignored when looking for rat anchor points.